Poland's outgoing prime minister Ewa Kopacz on Thursday announced the government's resignation after her liberal Civic Platform (PO) party lost a general election to conservative rivals Law and Justice (PiS).

Kopacz delivered a combative speech at the first sitting of the new parliament, telling the victorious PiS that her party "will not overlook any lies and we will hold you to your every promise."

The PO lost the October 25 election after eight years in power.

While the centrists' rule coincided with a period of strong growth and a fall in unemployment, many voters became disillusioned, believing it had wasted time on implementing reforms.

The departure last year of then PO leader Donald Tusk to Brussels, to take up the position of EU Council president, left the party further adrift.

"If the new leadership drives Poland out of the West and into the East, the Civic Platform will have a map ready to steer things back to normal," Kopacz told parliament.

Kopacz later handed her resignation to PiS-backed President Andrzej Duda.

She is expected to continue handling everyday government business until her successor Beata Szydlo is appointed.

Szydlo's cabinet could be sworn in on Friday or Monday, Poland's PAP news agency reported Thursday, quoting presidential sources.